Failures of a microprocessor device that occur during execution of code in the microprocessor under certain operating conditions must be discovered and analyzed during the development of that microprocessor so that the "bugs" in that microprocessor causing the failure can be corrected. The most difficult failures to track and solve are those that are "non-deterministic" and those that occur infrequently. A non-deterministic or infrequent failure must be verified and then analyzed to determine if it is a circuit characteristic, noise, or speed-path originating bug, for example.
A non-deterministic failure occurs randomly or pseudo-randomly and is difficult to identify as being caused by a specific circuit in the microprocessor. Often, non-deterministic type failures occur as chip operating temperatures or supply voltage levels are varied. Infrequently occurring failures are somewhat consistent, but do not exhibit themselves frequently during the execution of the code. Infrequent failures are often characterized by only occurring in specific applications such as booting operating systems or during the execution of a specialized software program, and they cannot be reproduced by using a simpler program or test vector. Also, these failures can occur thousands or even millions of instructions into the program, making it difficult to determine where in the program the failure occurred.
When performing a "debug" process of verifying and analyzing a failure, it would be desirable to analyze the internal state of the microprocessor not only at the point of the failure, but also at various times prior to that failure in order to understand the operation of the microprocessor leading up to the failure. However, in cases where the failures of interest are non-deterministic and infrequent, a significant amount of time may be required to place the microprocessor under test in the proper conditions to repeat the failure of interest. Similarly, many millions of lines of code may have to be executed in the microprocessor to reach the appropriate conditions or states of the processor to reach the infrequent failure. Because of these difficulties, debugging tools known in the art provide the internal state of the microprocessor at the point of failure, but the conditions of the processor leading up to the failure cannot be discerned from the provided data. Thus, to discover the state of the processor prior to the failure, the process is normally repeated and the processor halted prior to the failure. This, however, is generally impossible when the types of failures being considered are difficult to reproduce in a predictable way such as with non-deterministic and infrequently occurring failures. As can be seen, debugging tools in the prior art are not useful in providing important information about the operation of the processor leading up to the failure; yet, this information is critical for analyzing non-deterministic and infrequent failures and correcting bugs.